1. Field of the Invention
The present invention relates generally to automated loaded-board test probe fixture mechanisms, and more particularly to two-sided bed-of-nails clamshell probe fixtures.
2. Description of the Prior Art
The earliest printed circuit boards (PCBs) placed through-hole circuit devices on one side and had an interconnecting printed pattern of copper foil etched on the opposite side. Automated loaded-board testing was very simple because an array of spring-loaded probe pins, commonly known as a bed-of-nails, could be used to make certain strategic contacts with key points on the PCB. A low degree of circuit integration and a lack of crowding on early PCBs, together with the single sided construction, lasted for several years and the challenges presented to automated testing equipment (ATE) manufactures mainly involved signal simulation, interpretation and increasing microcomputer clock speeds.
A bed-of-nails test fixture is described by Everett in U.S. Pat. No. 4,357,062. A universal circuit board probe assembly is used to connect a variety of circuit board types to a diagnostic test probe fixture mechanism. The probe assembly is an integrated test probe fixture mechanism that includes a general purpose perforate platform with the holes in a uniform grid pattern. Opposing double-action movement floating connector pins are used with a removable backing plate that confronts the platform. A means for aligning the backing plate with the platform and circuit board test points is provided.
Vacuum fixtures have developed as a popular way to draw-down a PCB onto its matching bed-of-nails during a test. The use of vacuum permits an even and relatively non-stressful application of a large amount of force that can be needed, especially when the spring pressures of a hundred or more individual probes must be simultaneously overcome. For single-sided probing, vacuum fixtures allow topside access to a PCB, because a vacuum seal may usually be easily maintained.
In U.S. Pat. No. 4,344,033, Stowers, et al., describe a vacuum-actuated test fixture that has a yielding diaphragm assembly with a rigid frame and spring-loaded diaphragm base plate. A test fixture distributes downward atmospheric pressure evenly over the entire area of the board without localized stress concentrations. A captive removable diaphragm frame and quick-acting spring-loaded cam latches make a positive seal around the margins of the diaphragm assembly and along top and bottom edges. A plug-in vacuum adapter with manual valve provides for hook-up of a test fixture to a vacuum source. A lower base frame and patchboard allow interfacing of a test fixture with a receiver frame.
Surface mount devices (SMD) are now in common usage and each pin of an SMD cannot be expected to through-hole (via) to the PCB bottom. It therefore has become necessary to probe both sides of a PCB to gain access to what signals are available on an assembled PCB.
However, double-side probing vacuum fixtures obstruct access because a vacuum must be maintain on both sides of a PCB. Therefore, switches cannot be set, nor option jumpers changed, nor variable resistors adjusted while a PCB is being tested. Moreover, such double-sided vacuum fixtures are expensive to construct and slow down production times.